1. Field of the Invention
The present invention relates to solid-state imaging devices, methods for driving solid-state imaging devices, and imaging apparatuses. More specifically, the present invention relates to a charge-transfer solid-state imaging device, such as a charge-coupled-device (CCD) imaging device, a method for driving the solid-state imaging device, and an imaging apparatus, such as a digital still camera, including the charge-transfer solid-state imaging device as an imaging device.
2. Description of the Related Art
Imaging apparatuses, e.g., digital still cameras (DSCs), include all-pixel-readout solid-state imaging devices, such as CCD imaging devices, as imaging devices. In the all-pixel-readout solid-state imaging devices, signal charges of all pixels that are simultaneously read out to vertical transfer units are vertically transferred individually, rather than jointly, by the vertical transfer units, and are horizontally transferred and output by a horizontal transfer unit. The number of pixels in CCD imaging devices for DSCs has increased in order to increase the still image quality.
In CCD imaging devices including multiple pixels, smearing due to higher-density cells (or unit pixels) is conspicuous, and, in particular, is conspicuous in a moving-image capturing mode or a monitoring mode. Smearing is a phenomenon unique to CCD imaging devices in which vertical bright stripes appear in high-brightness areas of an image of an object when bright light enters vertical transfer units for transferring signal charges. The longer the signal charges remain in the vertical transfer units, the more conspicuous the smearing effect is.
The frame interline transfer (FIT) method that is used in professional broadcast CCD imaging devices is one solution for reducing the occurrence of smearing. In FIT-type CCD imaging devices, a light-shielded accumulator for temporarily accumulating signal charges transferred by vertical transfer units is provided below an imaging unit including a matrix of pixels. The signal charges are read out from the pixels to the vertical transfer units, and are then rapidly transferred to the accumulator by the vertical transfer units performing a high-speed transfer operation. The period of time during which the signal charges remain in the vertical transfer units is reduced, thereby reducing the occurrence of smearing.
However, such FIT-type CCD imaging devices with accumulators lead to a large chip size, which is about 1.5 to 2 times as large as the chip size of CCD imaging devices of the interline transfer (IT) type without accumulators. In view of cost, therefore, it is difficult to use FIT-type CCD imaging devices as imaging devices in consumer imaging apparatuses, such as digital still cameras.
While the VGA quality (640 pixels wide by 480 pixels high) is suitable for the DSC video function, demands for DSC CCD imaging devices including a large number of pixels have increased in order to increase the still image quality. With the demands for CCD imaging devices including more pixels, an increased number of pixels lead to a large gap between the frame rate in a still-image capturing mode and the frame rate used for the vide function (including monitoring).
One known implementation of the video function is a technique for thinning out signal charges read out from pixels in the vertical direction to increase the frame rate. For example, referring to FIG. 12, in color coding of two (horizontal) by two (vertical) pixel patterns, two pixels of each color for every 16 vertical pixels (16 lines) are read out to vertical transfer units and added in the vertical transfer units, and the signal charges of the remaining pixels are not read out (or are thinned out) (4/16-line readout).
In the vertical thinning-out and addition operation, signal charges of four pixels for every 16 pixels are read out to the vertical transfer units, and signal charges of 12 pixels are not read out, that is, 12 pixels are thinned out. In the vertical transfer units, packets of the read out signal charges (a packet is the unit in which charges are handled) and empty packets of the unread signal charges include smear components, and the smear components are added by charge transfer. Thus, although the signal components are thinned out, the number of smear components increases, and the occurrence of smearing increases.
Due to a high thinning-out rate, or a large number of pixels being thinned out, information regarding the thinned out pixels is not reflected in the final captured image, and false-color signals or moiré artifacts are caused. In addition, the amount of horizontal pixel information is excessively larger than the amount of vertical pixel information, which is uneconomical, and there is no balance between the vertical resolution and the horizontal resolution.
In the related art, the number of vertical pixels to be thinned out is reduced to increase the amount of pixel information, thereby preventing the occurrence of smearing or the generation of false-color signals. Furthermore, in order to prevent the horizontal driving frequency (the driving frequency of the horizontal transfer unit) from increasing due to an increased amount of pixel information, pixel addition is also performed in the horizontal direction to reduce the amount of pixel information (see, for example, Japanese Unexamined Patent Application Publication No. 11-234569).
For example, referring to FIG. 13, in color coding of two (horizontal) by two (vertical) pixel patterns, two pixels of each color for every eight vertical pixels (eight lines) are read out to vertical transfer units and added in the vertical transfer units (4/8-line readout), and two pixels of each color in the horizontal direction are added in a horizontal transfer unit. The number of pixels from which signal charges are not read out is therefore reduced to one third compared with 4/16-line readout, thereby reducing the occurrence of smearing or the generation of false-color signals.